1. Field of the Invention
The present invention relates to a semiconductor laser, a semiconductor laser driver and a method of driving a semiconductor laser. In particular, the present invention relates to a semiconductor laser, a semiconductor laser driver and a method of driving a semiconductor laser reducing feedback-induced noise by a modulated optical output.
2. Description of the Background Art
Semiconductor lasers are superior in monochromaticity and directivity and can provide a necessary spot size which is small enough to read a pit on an optical disk, and are accordingly used as a light source for the optical disk. For such a high-density recording medium as DVD (Digital Versatile Disk) having a multi-layer structure, a high output of at least 8 mW is necessary for reading and thus conventional red semiconductor lasers cannot be used for the high-density recording medium.
A nitride semiconductor laser with a short wavelength and a high output power is appropriate for the high-density recording medium like the DVD, and thus development of the nitride semiconductor laser, which is to be used as an effective laser for a laser pickup device instead of the red semiconductor laser, is being in progress.
With such a pickup device, light emitted from a laser is reflected from a surface of a disk which is a recording medium or from an optical system, and then the emitted light is partially returned to the laser. The light returned to the laser is called “feedback light.” The feedback light and the light emitted from the laser interfere with each other, resulting in noise generation when information is reproduced.
An effective method for reducing “feedback-induced noise” is to periodically change the intensity of an optical output and thereby reduce the coherency of the semiconductor laser. An optical output with its light intensity periodically changed is hereinafter referred to as “modulated optical output.”
In order to produce the modulated optical output, a technique of intentionally causing “self pulsation” of the laser is generally employed. The self pulsation can be caused by providing, for example, the laser with such a special structure as described below.
Around a gain region within an active layer that is called light-amplifying region, a region having a light-absorbing effect that is called saturable absorber region is formed. Accordingly, on carriers in the active layer and oscillating light, the Q-switching effect is exerted by the saturable absorber region to cause the self pulsation.
Thus, a constant current equal to or larger than a lasing threshold can be injected into the active layer of the laser with the above-described structure so as to intentionally cause the self pulsation and thereby obtain a modulated optical output.
FIG. 11 is a cross-sectional view exemplarily showing a structure of a conventional self-pulsating semiconductor laser.
The conventional self-pulsating semiconductor laser device shown in FIG. 11 includes, on an n-type GaAs substrate 103, an n-type GaAs buffer layer 104, an n-type AlGaInP cladding layer 105, a GaInP active layer 106, a p-type AlGaInP cladding layer 107, a p-type GaInP intermediate layer 108 and a p-type GaAs contact layer 109 that are deposited successively in this order.
P-type AlGaInP cladding layer 107, p-type GaInP intermediate layer 108 and p-type GaAs contact layer 109 constitute a stripe ridge 110, and n-type GaAs buried layers 111 and 112 are provided respectively on lateral sides of ridge 110. A p-electrode 101 is provided on the front side of the device and an n-electrode 102 is provided on the back side thereof that are not divided.
This device has ridge 110 in cladding layer 107 located on active layer 106 so as to produce regions into which currents of different amounts respectively are injected, and thus saturable absorber regions 114 and 115 are provided around a light-amplifying region 113 (gain region) to achieve the self pulsation.
Here, a current is injected from p-electrode 101 through the inside of ridge 110 into the active layer 106 and then passed to n-electrode 102. The current is chiefly injected into a region directly below the ridge that accordingly functions as a light-amplifying region 113 while a smaller amount of current is injected into regions 114 and 115 that accordingly function as saturable absorber regions. In this way, the self pulsation is caused.
For such a device, various parameters of the device structure that can cause the self pulsation, for example, the width of the ridge structure and the thickness of the cladding layer, should be determined. Moreover, unless the device is precisely produced to satisfy the determined conditions, the self pulsation is impossible to achieve.
A conventional nitride semiconductor laser device is shown next that has a region into which a smaller amount of current is injected, i.e., saturable absorber region, as the conventional device shown in FIG. 11.
FIG. 12 is a cross-sectional view showing a structure of the conventional nitride semiconductor laser device disclosed in Japanese Patent Laying-Open No. 2000-286504.
The conventional nitride semiconductor laser device shown in FIG. 12 is produced by depositing, on a sapphire substrate 121, an n-type contact layer 122 of GaN having a thickness of 4.5 μm, an n-type cladding layer 123 of AlbGa1−bN having a thickness of 0.8 μm, an active layer 124 having a multiple quantum well structure, a p-type cladding layer 125 of AlaGa1−aN having a thickness of 0.8 μm, n-type current-constricting layers 126 and 127 of AlcGa1−cN, and a p-type contact layer 128 of GaN having a thickness of 0.05 μm.
P-type cladding layer 125 is constituted of a flat portion 125a formed to cover a surface of active layer 124 and a two-stripe portion constituted of a lower stripe portion 125b and an upper stripe portion 125c protruding upward from the central part of flat portion 125a. The top surface of the two-stripe portion contacts p-type contact layer 128 and the lateral sides thereof contact n-type current-constricting layers 126 and 127. Here, the width of upper stripe portion 125c is made smaller than the width of lower stripe portion 125b. 
On n-type contact layer 122, an n-electrode 129 is formed. On p-type contact layer 128, a p-electrode 130 is formed. In active layer 124, well layers of InxGa1−x N having a thickness of 8 nm and barrier layers of InyGa1−yN having a thickness of 16 nm are stacked alternately. In the device structure shown in FIG. 12, n-type current-constricting layers 126 and 127 made of AlcGa1−cN has a refractive index which is made smaller than that of p-type cladding layer 125 made of AlaGa1−aN (0<a<c≦1) and thereby a real-refractive-index waveguide structure is implemented.
This conventional nitride semiconductor laser device efficiently uses, as a saturable absorber region, the region into which a smaller amount of current is injected. Then, the ridge is formed so that the width of the ridge for current injection and the width of a region where a transverse mode of laser light spreads are different from each other. Neither P-electrode 130 nor n-electrode 129 is divided. P-electrode 130 is formed on the top surface of p-type contact layer 128 while n-electrode 129 is formed on the top surface of n-type contact layer 122 and they are provided as a pair of electrodes.
According to the above-discussed method, a region into which a smaller amount of current is injected is used as the saturable absorber region. Alternatively, a saturable absorber region and a light-amplifying region may be provided in advance in fabricating the device. In this case, the carrier lifetime and the differential gain are adjusted by adding impurities so as to satisfy the parameter conditions for causing the self pulsation and accordingly, a layer serving as the saturable absorber region and a layer serving as the light-amplifying region are produced. Such a conventional self-pulsating semiconductor laser is shown in FIG. 13.
FIG. 13 is a cross-sectional view showing a structure of a self-pulsating semiconductor laser device disclosed in Japanese Patent Laying-Open No. 8-204282.
The conventional self-pulsating semiconductor laser device shown in FIG. 13 includes an n-electrode 141, an n-type GaAs substrate 142, an n-type AlGaInP cladding layer 143, an n-type AlGaInP saturable absorber layer 144, an n-type AlGaInP cladding layer 145, an AlGaInP active layer 146, a p-type AlGaInP cladding layer 147, a p-type AlGaInP saturable absorber layer 148, a p-type AlGaInP cladding layer 149, n-type GaAs current-blocking layers 150 and 151, a p-type GaAs contact layer 152 and a p-electrode 153.
This laser device has saturable absorber layers 144 and 148 in itself that have the light-absorbing effect, and carriers in these saturable absorber layers 144 and 148, and carriers and oscillating light in active layer 146 which is a light-amplifying region cooperate with each other to cause self-pulsation. The laser satisfying the self-pulsating condition can produce an optical output with a periodical change in intensity, which is achieved by injection of direct current into the light-amplifying region.
When the above-discussed method is used to cause the self pulsation, however, the composition of the device and conditions of the structure thereof are limited. For example, in order to fabricate saturable absorber layers 144 and 148 and active layer 146 which is a light-amplifying region by adding impurities thereto, the carrier lifetime and the differential gain of the saturable absorber regions and the light-amplifying region should be adjusted.
According to the methods as described above, the intensity of an optical output is periodically changed by using the self pulsation. There is another method for producing a modulated optical output, which is specifically a high-frequency superimposition method according to which a current modulated by the high frequency is injected. For example, according to a technique disclosed in Japanese Patent Laying-Open No. 60-35344, pulsed lasing is caused by superimposing a modulation current of high frequency on an operating current near a lasing threshold of a laser.
FIG. 14 illustrates operational characteristics of the conventional semiconductor laser disclosed in Japanese Patent Laying-Open No. 60-35344.
In FIG. 14, (a) indicates an injection-current to optical-output characteristic curve of the conventional semiconductor laser, with the horizontal axis representing an injection current and the vertical axis representing an optical output produced according to the injection current. To the semiconductor laser having such an injection-current to optical-output characteristics, a current modulated in a range between a current value smaller than a lasing threshold Ith and a current value higher than the lasing threshold Ith as indicated by (b) in FIG. 14 is injected, so that an optical output of the semiconductor laser is modulated as indicated by (c) in FIG. 14.
In this case, it is unnecessary to cause the self pulsation state and thus unnecessary to form a structure having the saturable absorber region and light-amplifying region in fabricating the device. Moreover, the amplitude of the modulation current to be injected can be increased to increase the amplitude of a resultant modulated optical output.
There is a further method of producing a modulated optical output, according to which a semiconductor laser in a bistable state with a light-amplifying region and a saturable absorber region is provided and a current or voltage to be applied to the saturable absorber region is varied.
FIG. 15 shows characteristics of an injection current injected into a light-amplifying region vs. an optical output of a conventional semiconductor laser in a bistable state.
As shown in FIG. 15, the conventional semiconductor laser in the bistable state exhibits hysteresis characteristics in the relation between the injection current and the optical output. The solid line indicated by A is herein referred to as lower hysteresis path and the dotted line indicated by B is herein referred to as upper hysteresis path.
Referring to FIG. 15, as a current is injected into the light-amplifying region only, the optical output increases from P4 through P1 along the path indicated by A. At this time, the carrier density in the saturable absorber region increases because of absorption of light generated in the light-amplifying region, and accordingly, the light-absorbing effect in the saturable absorber region decreases.
Then, as the amount of a current injected into the light-amplifying region increases, the light-absorbing effect reaches saturation. Then, when the injection current attains IthON, the optical output suddenly increases from P1 to P2. This IthON is herein referred to as rising threshold of hysteresis.
Then, as the amount of the injection current is decreased, the optical output gradually decreases from P2 through P3 along the path indicated by B, since the saturable absorber region cannot immediately recover the light-absorbing effect and thus the optical output does not dramatically decrease. At this time, as the carrier density as well as the optical output decrease, the light absorbing effect of the saturable absorber region is recovered.
Then, as the amount of the injection current injected into the light-amplifying region is decreased, the light-absorbing effect is sufficiently recovered so that the optical output sharply decreases from P3 to P4 when the injection current reaches IthOFF. IthOFF is herein referred to as falling threshold of hysteresis.
The shape of the hysteresis paths changes according to voltage application or current injection to the saturable absorber region. When a voltage is applied to or a current is injected into the saturable absorber region, the carrier density increases and the light-absorbing effect decreases. Then, the whole hysteresis shifts to a region where the injection current is lower, so that IthON decreases. In this way, by increasing/decreasing the light-absorbing effect of the saturable absorber region, the lasing threshold can be changed. A conventional driving method of this type as described above is shown in FIGS. 16 and 17.
FIG. 16 is a schematic cross-sectional view showing a structure of a conventional semiconductor laser device in a bistable state disclosed in Japanese Patent Laying-Open No. 2-137383.
The conventional bistable semiconductor laser shown in FIG. 16 is a transverse-mode-controlled semiconductor laser of AlGaAs/GaAs formed on a GaAs substrate. One of electrodes of the laser is divided to provide a light-amplifying region 161 and a saturable absorber region 162 in an active layer 163. Then, according to voltages V1 and V2 applied to saturable absorber region 162, a lasing threshold current injected into light-amplifying region 161 is varied. In other words, a bias current IB applied to light-amplifying region 161 is set to an intermediate value of lasing thresholds according to voltages V1 and V2, and a signal voltage V which changes between V1 and V2 is applied to saturable absorber region 162.
FIG. 17 shows an injection current—optical output characteristic curve of the conventional bistable semiconductor laser shown in FIG. 16.
Since saturable absorber region 162 which is a loss for oscillating light is provided in active layer 163, the optical output non-linearly increases at a current value, when a current is injected into light-amplifying region 161 only. This lasing threshold current Ih1 or Ih2 changes according to the amount of light absorbed in saturable absorber region 162. The threshold current decreases as the amount of absorbed light increases. When the voltage applied to the saturable absorber region increases from V1 to V2 (V1<V2), the amount of absorbed light in saturable absorber region 162 accordingly increases so that the lasing threshold current decreases from Ih1 to Ih2.
Thus, as shown in FIGS. 16 and 17, a certain bias current IB (Ih2<IB <Ih1) is injected into light-amplifying region 161 while a signal voltage V changing from V1 to V2 is applied to saturable absorber region 162, so that the lasing threshold current varies between Ih1 and Ih2 to produce a modulated optical output P.
The conventional art shown in FIGS. 11-13 according to which a modulated optical output is produced by using self pulsation has a problem that the composition and structure of the device are limited in fabricating the device.
If the composition in fabricating the device is utilized to provide a saturable absorber region and a light-amplifying region, the ratio between the saturable absorber region and the light-amplifying region in terms of carrier lifetime as well as in terms of differential gain must be adjusted. However, the range of parameters satisfying the self pulsation condition is narrow so that the freedom of fabrication is lessened. In particular, the nitride semiconductor laser shown in FIG. 12 confronts a serious problem that the value of the differential gain cannot be changed to a great degree due to characteristics of the nitride.
In general, the balance of the carrier density between the saturable absorber region and the light-amplifying region determines whether the semiconductor laser enters a self-pulsating state or bistable state. The balance can be adjusted by using the ratio in terms of the length in the direction of the resonator, the ratio in terms of the carrier lifetime and the ratio in terms of the differential gain, between these regions. In a laser in the self-pulsating state, the carrier lifetime of the light-amplifying region should be longer than that of the saturable absorber region and the differential gain of the light-amplifying region should be smaller than that of the saturable absorber region. Then, it is necessary that the ratio in terms of the differential gain and the ratio in terms of the carrier lifetime are in specific ranges respectively.
Specific values of these parameters vary depending on characteristics of a semiconductor. For a nitride semiconductor laser, the ratio of the differential gain between the saturable absorber region and the light-amplifying region must be made larger than that of a GaAs-based red semiconductor laser for example. It is noted that, if there is a greater difference between the saturable absorber region and the light-amplifying region in terms of the gradient of a gain characteristic curve with respect to the carrier density, namely the differential gain, the self pulsation is more easily caused.
More specifically, a greater gradient of a gain characteristic curve with respect to the carrier density in the saturable absorber region makes it possible to change the carrier density with less absorption of light. Accordingly, the carrier density can be changed easily. A great change in carrier density in the saturable absorber region causes a change in light-absorbing effect. For the nitride semiconductor laser, however, it is extremely difficult to provide different differential gains respectively of the saturable absorber region and the light-amplifying region due to characteristics of the nitride.
FIG. 18 shows gain characteristic curves in a saturable absorber region and a light-amplifying region of semiconductor lasers. The solid line represents gain characteristics of a GaAs semiconductor laser and the broken line represents gain characteristics of a GaN semiconductor laser.
Carriers of the GaN semiconductor laser have a larger effective mass than that of carriers of a red semiconductor laser. Regarding the GaN semiconductor laser, as shown in FIG. 18, the difference in gradient between the section of the gain characteristic curve in the saturable absorber region (absorbing region) and that in the light-amplifying region (gain region) is smaller than that of the GaAs semiconductor laser which is a red semiconductor laser. Therefore, the ratio between differential gains respectively in those regions represented by the inclination of the gain characteristic curve is approximately 1 and thus the laser cannot satisfy the self-pulsating condition. Moreover, the nitride semiconductor laser including the GaN semiconductor laser has a problem that it is difficult to change the differential gain by addition of impurities, in contrast to the red semiconductor laser.
For the reason above, it is difficult to produce the nitride semiconductor laser by adjusting the carrier lifetime and the differential gain so as to allow the saturable absorber region and the light-amplifying region to satisfy the self-pulsating condition. No method has been found to surely cause the self pulsation.
In addition, as shown in FIG. 12, when the ridge is provided in the structure of the nitride semiconductor laser so as to produce regions into which different amounts of currents are injected and thereby produce a saturable absorber region and a light-amplifying region, numerous and detailed conditions in terms of the structure, such as the width and thickness of the ridge, the thickness of the multi-layer along the boundary of the ridge, the thickness of the cladding layer and etching conditions, must be optimized. Moreover, technique for precisely fabricating the device under the resultant conditions is necessary.
Then, in order to achieve the self pulsation by the structure as shown in FIG. 12, many conditions should be defined first. Even if the conditions are established, there remains a problem that the yield of products is poor due to unsatisfactory technique of fabricating the devices. Thus, regarding the nitride semiconductor laser, there has been no method for manufacturing a laser device producing the self-pulsating state.
Regarding the method as shown in FIG. 14 that is a high-frequency superimposing method for obtaining a modulated optical output without using the self-pulsating state, a great amplitude of a modulation current to be injected is necessary for producing an optical output with an amplitude required for achieving the effect of reducing noise. A resultant problem is that, when a great optical output which is necessary to be applied to a pickup device for a high-density recording medium is produced, power consumption and generated heat increases.
Further, regarding the method as shown in FIG. 15 of increasing/decreasing the voltage to be applied to the saturable absorber region of a bistable semiconductor laser and increasing/decreasing the resistance thereof so as to adjust the lasing threshold of the laser and thereby produce a modulated optical output, it is necessary to apply a voltage or current of a large amplitude to the saturable absorber region in order to produce an optical output of a large amplitude which is enough to reduce feedback light. However, if the voltage or current of a large amplitude is applied to the saturable absorber region, a high voltage or current must be applied accordingly.
Then, when a modulated optical output is to be produced by the method as shown in FIG. 15, it is likely that the saturable absorber region saturates and thus an optical output has a small change in intensity. As a result, the optical output has a small amplitude and further, a pulsed optical output is hard to produce. Accordingly, the coherence is difficult to reduce, which lowers the effect of removing feedback-induced noise.
The problems of conventional techniques discussed above are summarized as follows.
For such a high-density recording medium as DVD which has a multi-layer structure, an output of at least 8 mW is required even for reading. Therefore, conventional GaAs-based self-pulsating semiconductor lasers cannot be used for such a medium.
Instead of the conventional semiconductor lasers, a nitride semiconductor laser is now being developed for use as a next-generation device. Although the nitride semiconductor laser produces a high output, this laser is difficult to produce due to a narrow range of conditions for achieving the self-pulsating state. For a conventional semiconductor laser having a ridge in its structure for providing a saturable absorber region and thereby producing the self-pulsating state, it is difficult to specify conditions of the structure. Moreover, the self-pulsating state cannot be produced unless the device is precisely fabricated, and thus the yield is poor.
For a conventional semiconductor laser into which an injection current modulated to be temporarily lower than the lasing threshold is injected so as to obtain a pulsed optical output, if the maximum value of the current is increased for producing an optical output having an amplitude which is large enough to reduce feedback-induced noise, the amplitude of the modulation current must be increased, resulting in a problem of a greater load on a modulating circuit.
Moreover, for a conventional semiconductor laser for which a voltage to be applied to the saturable absorber region of the semiconductor laser in the bistable state is increased/decreased in order to modulate an optical output, the lasing threshold of the laser must be switched with a wide range for producing a high optical output necessary for a pickup device of a high-density recording medium. Then, as the lasing threshold is increased, the value of current to be injected into the light-amplifying region should be increased.